Second harmonic generation induced by charge-transfer complexes in a centrosymmetric medium

We consider the second harmonic generation (SHG) caused by impurities embedded in a nonlinear (χ⁽³⁾) optical medium which itself does not display χ⁽²⁾ optical response. In the case when the impurity molecules (charge-transfer (CT) complexes) possess an optical transition accompanied by charge transfer, we demonstrate that the excitation of impurities with a pump field may result in a contribution of the host medium to SHG. Depending on the presence or absence of the angular alignment of the excited static CT dipoles, there may be coherent SHG or weak incoherent SH scattering.     

Second harmonic generation in polymers containing dithiosquarylium dye

Polymer layers containing the dye 2,4-bis(1,3,3-trimethyl-2-indolinylidenemethyl)-1,4-dithiosquaraine display second-order nonlinear optical properties. Depending on conditions of preparation of the layers, the monomer form of the dye or the intermolecular charge transfer complex between two dye molecules are responsible for the second harmonic generation. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 86–91, January, 1999.     

Second harmonic generation in planar samples of ferroelectric liquid crystals

Non-linear optical properties of ferroelectric liquid crystals are often studied through second harmonic generation measurements on homeotropic cells. However, it is not always possible to prepare well aligned unwound samples in the homeotropic geometry, especially when dealing with photopolymers or tightly twisted SmC* materials. Here we analyse the problem of extracting the second order susceptibility coefficients dij from second harmonic generation measurements performed on planar samples. Several experimental configurations of polarizer, sample and analyser dispositions are proposed to deduce the complete dij tensor together with the dispersion of the refractive indices and the birefringence both at the fundamental and second harmonic frequencies. Measurements carried out on planar and homeotropic cells for a classical non-linear optical material (W314) are presented and the resulting dij coefficients are compared.     

Second harmonic generation in ferroelectric liquid crystals and their mixtures

The second harmonic generation (SHG) in the ferroelectric liquid crystal (FLC) state has been studied as functions of electric field strength, rotating angle, temperature and molecular structure. It has been confirmed that a sharp angularly phase-matching curve of the SHG controlled by an electric field is observed even in the liquid crystal. The temperature dependences of the phase-matched SHG and Maker fringe in the ferroelectric phase have also been studied, and temperature dependences of non-linear optical coefficients obtained. The SHG in several kinds of FLC and dye doped FLC have also been measured, and the enhancement of SHG realized by means of doping the FLC with several kinds of dye.     

Second harmonic generation in ferroelectric liquid crystals and their mixtures

Abstract

The second harmonic generation (SHG) in the ferroelectric liquid crystal (FLC) state has been studied as functions of electric field strength, rotating angle, temperature and molecular structure. It has been confirmed that a sharp angularly phase-matching curve of the SHG controlled by an electric field is observed even in the liquid crystal. The temperature dependences of the phase-matched SHG and Maker fringe in the ferroelectric phase have also been studied, and temperature dependences of non-linear optical coefficients obtained. The SHG in several kinds of FLC and dye doped FLC have also been measured, and the enhancement of SHG realized by means of doping the FLC with several kinds of dye.     

Second harmonic generation and polar order in thin films of polyesters

A series of optically active main chain polyesters were synthesized by high temperature polycondensation from biphenolic azo chromophores with azobenzene-4,4'-dicarbonylchloride and 1,4:3,6-dianhydro-D-sorbitol ([!]_D²⁵=+42.5°). The second harmonic generation of the polyesters is studied in thin films. The polar order in films is determined from the UV-visible absorption spectra. The variation in SHG intensity with chiral monomer concentration is explained as a function of polar order. The polymers with chiral units showed good second harmonic efficiency. The intensity of second harmonic light (532 nm) increased with angle of incidence and indicated the polar order in the plane of the film. The SH signal also showed good temporal stability.     

Nonlinear optical polymers. Second harmonic generation in corona-poled thin films

The synthesis and second harmonic coefficients, d_3,1 and d_3,3 as well as the related susceptibilities χ⁽²⁾_zzz of five series of (NLO-dye methacrylate)-(methyl methacrylate) copolymers were investigated. The NLO-chromophores bound covalently to the polymer backbone were 5-(2,2-dicyanovinyl)-or 4-(2-cyano-2-methoxycarbonyl)vinyl-1-piperidino-2-thiophene (P₁ and P₂), 4-nitro-4′-alkoxy-stilbene (P₃), 4-nitro-3′-methoxy-4′-alkoxystilbene (P₄) and 4-nitro-4′-alkoxy-α-cyano stilbene (P₅). The second order nonlinear optical properties of corona-poled aligned thin polymer films, using a needle electrode in order to induce noncentrosymmetry, were evaluated. Nonlinear susceptibilities, χ⁽²⁾_zzz, of the films were derived from the analysis of full-angle Maker fringe patterns at 1064 nm, χ⁽²⁾_zzz values as high as 1.98×10⁻⁷ esu for P₂ copolymers and of 1.19×10⁻⁷ esu for P₃ copolymers could be achieved.     

Second harmonic generation and confined acoustic phonons in highly excited semiconductor nanocrystals

The photo-induced enhancement of second harmonic generation and the effect of nanocrystal shape and pump intensity on confined acoustic phonons in semiconductor nanocrystals have been investigated with time-resolved scattering and absorption measurements. The second harmonic signal showed a sublinear increase of the second-order susceptibility with respect to the pump pulse energy, indicating a reduction of the effective one-electron second-order nonlinearity with increasing electron-hole density in the nanocrystals. The coherent acoustic phonons in spherical and rod-shaped semiconductor nanocrystals were detected in a time-resolved absorption measurement. Both nanocrystal morphologies exhibited oscillatory modulation of the absorption cross section, the frequency of which corresponded to their coherent radial breathing modes. The amplitude of the oscillation also increased with the level of photoexcitation, suggesting an increase in the amplitude of the lattice displacement as well.     

Second harmonic generation and optical anisotropy of a spin-cast polymer film

The nonlinear susceptibilities (χ⁽²⁾₃₃, χ⁽²⁾₃₁), and the refractive index anisotropy, of a main chain accordion polymer, BCSC (see the text for the complete chemical formula), have been measured by the techniques of second harmonic generation (SHG) and waveguiding. A large birefringence, which is induced in the BCSC film by the spin-coating process, is almost removed by electric field poling. Our results demonstrate the fundamental difference between the birefringence associated with quadrupolar orientational order and the dipolar order associated with SHG. © 1993 John Wiley & Sons, Inc.     

Aligned inclusion of hemicyanine dyes into silica zeolite films for second harmonic generation

Silicalite-1 films (thickness = 400 nm) supported on both sides of glass plates (SL/G) were prepared, and hemicyanine dyes (HC-n) with different alkyl chain lengths (n, n = 3, 6, 9, 12, 15, 18, 22, and 24) were included into the silicalite-1 films by dipping SL/Gs into each methanol solution of HC-n (1 mM) for 1 d. The included numbers of HC-n per channel (N(C)) generally decreased with increasing n; that is, they were 6.4, 23.1, 15.4, 8.2, 5.7, 3.5, 0.9, and 1.2 molecules per channel, respectively. The d(33) value gradually increased with increasing n but decreased when n > 18; that is, they were 1.12, 0.50, 2.25, 3.59, 4.99, 5.30, 1.71, and 2.57 pm V(-1), respectively. However, d(33)/N(C) progressively increased with increasing n. The d(31) values were approximately 100 times smaller than the corresponding d(33) values, and the average d(33)/d(31) ratio was 109, which is higher than those of Langmuir-Blodgett (LB) films and poled polymers of nonlinear optical (NLO) dyes, by approximately 2-5 and approximately 30-50 times, respectively. The estimated average tilted angle of the dyes with respect to the channel direction was 7.7 degrees, and the calculated average order parameter was 0.97, which is approximately 480 times higher than the values observed from poled polymers. The degree of uniform alignment (DUA) generally increased with increasing n. The progressive increase of both DUA and d(33)/N(C) with n is attributed to the increase in the tendency of HC-n to enter hydrophobic silicalite-1 channels with the hydrophobic alkyl chain first. A more than 134-fold increase in DUA was observed upon increasing n from 6 to 24. The DUA of HC-24 in the silicalite-1 film reached close to 1. Although the observed d(33) values were lower than those of the LB films of NLO dyes due to very small dye densities of the silicalite films, this methodology bears a great potential to be developed into the methods for preparing practically viable NLO films.     

Second harmonic generation in polymers containing a new azo chromophore based on phenylnitrobenzoxazole

The synthesis of the new chromophores 2‐{4‐[4‐(N,N‐dihydroxyethylamino)‐phenylazo]‐phenyl}‐6‐nitrobenzoxazole ( 1 ) and 4‐[4‐(N,N‐dihydroxyethylamino)phenylazo]‐benzoic acid ( 2 ) is described. Three homopolymers obtained by the polycondensation of 1 with propyloxy‐terephthalic acid and 1 and 2 with 2,4‐tolylenediisocianate as well as two copolymers obtained by polycondensation of appropriate mixtures of chromophores 1 and 2 with 2,4‐tolylene‐diisocianate were prepared and characterized. All as‐prepared polymers but one (homopolyurethane of 1 ) were amorphous and exhibited good optical clarity, good thermal stability above the glass transition, and solubility in organic solvents. The second‐order nonlinear optical behavior of the amorphous polymers was investigated on spin‐coated corona‐poled films through second harmonic generation (SHG) experiments at 1064 nm. At this wavelength, resonance‐enhanced values of the SHG coefficient d₃₃ were found in the range between 6 and 34 pm/V where higher values were observed for polymers with a higher molar content of 1 . © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1468–1475, 2002     

Second harmonic generation measurements in aligned samples of liquid crystals composed of bent-core molecules

The second-order susceptibility tensor of three liquid crystals composed of banana-shaped molecules has been fully determined. The measurements were carried out in single domain samples, aligned by means of an electric field parallel to the cell surface. Special care was taken with the appropriate determination of the index mismatch between the fundamental and second harmonic waves. This was achieved by using samples of different thickness. This parameter is very important for obtaining reliable values of the second order susceptibility tensor components, and is seldom directly measured. The suitability of the experimental procedure was checked by comparing the results obtained for the reference compound P-8-O-PIMB with previously reported measurements for this material. The d_ij coefficients obtained in this work are in the range 1–4?pm?V^?1 and are compared with other values obtained in previous reports.     

Second harmonic generation measurements in aligned samples of liquid crystals composed of bent-core molecules

The second-order susceptibility tensor of three liquid crystals composed of banana-shaped molecules has been fully determined. The measurements were carried out in single domain samples, aligned by means of an electric field parallel to the cell surface. Special care was taken with the appropriate determination of the index mismatch between the fundamental and second harmonic waves. This was achieved by using samples of different thickness. This parameter is very important for obtaining reliable values of the second order susceptibility tensor components, and is seldom directly measured. The suitability of the experimental procedure was checked by comparing the results obtained for the reference compound P-8-O-PIMB with previously reported measurements for this material. The d_ij coefficients obtained in this work are in the range 1-4 pm V^-1 and are compared with other values obtained in previous reports.     

Second harmonic generation applying injection-locked radiation from a high-power,broad-stripe laser diode

406 nm laser light is produced by second harmonic generation in a LiIO₃ crystal using the fundamental radiation of an injection-locked, broad-stripe, high-power laser diode. The efficiencies of injection locking as well as of second harmonic generation are discussed.     

Zeolite inclusion complexes

Zeolites provide the largest and most useful family of porous host crystals, stable in the presence or absence of guest molecules. An account is given of their structural characteristics in terms of intracrystalline channels and cavities of molecular dimensions; their total intracrystalline pore volumes; and the intracrystalline distribution patterns of guest molecules, often present as clusters or filaments. The way in which the shape and size of the molecules relate to the shape and size of the apertures or windows giving access to the cavities, together with other factors such as cation location, then determine molecule sieving behaviour, which is discussed with reference to several types of zeolite sieve. The relation between diffusivity and molecular dimensions is also illustrated. A comparison between zeolites and clathrate host lattices shows that there are strong similarities of several kinds. Zeolites may form inclusion complexes with metals, salts and polar or non-polar molecules, examples of which are briefly considered, together with isotherms for some of these types of complex. An account of equilibrium and its formulation demonstrates the thermodynamic background and illustrates the degree of success which has been obtained in describing isotherms in terms of various models.     

Second harmonic generation phase measurements of Cr(VI) at a buried interface

Surface second harmonic generation (SHG) phase measurements are carried out on methyl ester-functionalized fused quartz/water interfaces in the presence and absence of Cr(VI). The experiments are performed at pH 7, room temperature, and a chromate concentration of 10(-4) M, which corresponds to monolayer Cr(VI) coverage. The liquid/solid interface is probed from the fused quartz side by directing the probe light field at 580 nm onto the interface together with an SHG reference signal at 290 nm that is collinear with the fundamental. The phase difference of the SHG signals generated at the interface in the presence and absence of Cr(VI) is 85 degrees, which is consistent with SHG resonance enhancement observed for the surface-bound Cr(VI) near 290 nm. The optical arrangement discussed here does not require vacuum technology or optics that compensate for the dispersion of the fundamental and the second harmonic E-fields in the two condensed-phase media. This approach is general and can be applied for analyzing thermodynamic and kinetic data derived from SHG measurements of physical and chemical processes occurring at any buried interface.     

Second harmonic generation at the interface of copper tetra-tert-butyl phthalocyanine Langmuir-Blodgett film/metal

As one of the most primitive nonlinear optical phenomena, optical second harmonic genera-tion (SHG) has been investigated for half a century and it has become a very useful spectroscopic tool in the study of surface and interface[1—3]. It was theoretically shown that the SH signal cannot be generated in a centrosymmetric system. However, the generation of SH signal from the centro-symmetric molecules such as fullerene (C60) and CuPc has been detected[4—7]. In our recent ex-periments, an …     

Second harmonic generation (SHG) investigations of different phases of banana shaped molecules

New liquid crystals consisting of non-chiral, banana shaped molecules are investigated by the generation of the second harmonic (SHG) of an optical wave. The temperature and electric field dependence of the SH activity of the apparent thermodynamic phases of several homologues are studied and first quantitative values of the effective non-linear optical coefficients are given.